RSS enhancements

[neilv-g]

1. Deep Feature Categorization: Expand rssVisitor to categorize highlights into Baseline Newly Available, Widely Available, and Regressed to Limited Availability.
2. Browser Implementation Updates: Expand rssVisitor to extract and group browser version changes (e.g. Chrome 125, Safari 17.4).
3. Rich HTML Template & Permalinks: Update rssItemTemplate in rss_renderer.go to render feature permalinks (/features/{id}), MDN links, and styled baseline status badges.
4. Shared Summary Presentation Core: Extract a shared summary presentation package (lib/summaryrender) used by Email, Webhooks, and RSS to eliminate duplicated category and browser name formatting logic.

See the end of this comment.

5. fix(api): correct RSS JSON extraction and trigger filtering #2594 (comment)